Indirect fired heaters are generally used in applications where a clean particulate-free stream of heated air is required. Typical uses of these heaters include paint bake ovens, dry off ovens, and food processing ovens, as well as for curing wood, heat treating metals and general ventilation heating. These heaters consist of an enclosure having an inlet and an outlet, a heat exchanger within the enclosure for transferring thermal energy from gas within the heat exchanger to the air within the enclosure, a burner for heating the gas within the heat exchanger and a fan in the enclosure for moving air through the enclosure. In operation, the fan draws air into the enclosure through the inlet, propels the air through the heat exchanger where it is heated and exhausts the air out the outlet to be used.
In a typical indirect heater, an air-to-air shell and tube heat exchanger is used to transfer heat from a burner associated with the exchanger to the air flowing through the enclosure. The heat exchanger has a plurality of hollow tubes bundled together defining a plurality of gas passages within the exchanger that are spaced apart to allow air within the enclosure to flow around the outside of the tubes. In operation, hot gaseous combustion products from the burner make a single pass through the heat exchanger and are exhausted through a flue gas outlet. As the hot gaseous combustion products pass through the interior of the heat exchanger tubes, thermal energy is transferred to the air in the enclosure flowing through the heat exchanger around the exterior of the tubes.
Unfortunately, the single pass design of this heat exchanger construction results in a great deal of wasted heat being exhausted out the flue outlet, significantly lowering the operating life and efficiency of the heat exchanger. For example, normal flue discharge temperatures for an exchanger of this construction are extremely high, typically ranging between 1400.degree.-1500.degree. F., which increases the cost to operate the heater. The high temperature of the flue gases flowing through the heat exchanger tubes also causes increased corrosion within the tubes reducing the operating life of the heat exchanger. The extreme heat that the heat exchanger is subjected to during operation further magnifies the detrimental effect of thermal cycling upon the exchanger which can accelerate tube metal fatigue possibly leading to the premature failure of one or more heat exchanger tubes. As a result of the combined effects of increased corrosion and accelerated metal fatigue, heat exchanger failure approximately every 2 to 3 years is not uncommon, requiring expensive servicing and replacement of the heat exchanger.
The enclosure housing the heat exchanger is typically of airtight construction and insulated to reduce heat transmission through the walls of the enclosure. However, the inside and outside surfaces of the walls of the enclosure are both supported by a frame that acts as a heat conductive path through each wall of the enclosure. This heat loss can be quite substantial, lowering the overall operating efficiency of the heater while undesirably increasing the temperature of the outside surface of the enclosure walls. Should a nearby worker contact the outer enclosure surface, they could be severely burned or otherwise injured while distracted by the hot outer surface.